Atmospheric Pressure Ionisation (“API”) sources are commonly used to interface liquid chromatography to a mass spectrometer. There are many types of API sources, including electrospray (“ESI”), atmospheric pressure chemical ionization (“APCI”) and impactor spray (“IS”) sources.
FIG. 1 shows schematically a conventional standard impactor spray source. This comprises a pneumatic nebulizer assembly 1, a desolvation heater 4, an impactor target 5 and a mass spectrometer inlet assembly comprising cone gas nozzle 6, ion inlet orifice 8 and first vacuum region 9.
This arrangement may be surrounded by a source enclosure that contains an exhaust outlet for the venting of solvent fumes (not shown). The nebuliser assembly 1 is composed of an inner liquid capillary 2 and an outer gas capillary 3 which delivers a high velocity stream of gas at the nebulizer tip to aid the atomization of the liquid solvent flow. The inner liquid capillary 2 may have an internal diameter of 130 μm and an outside diameter of 270 μm. The outer gas capillary 3 may have an inside diameter of 330 μm.
The gas supply (for example nitrogen) is pressurized to approximately 7 bar and liquid flow rates of 0.1 to 1 mL/min are commonly used. A heated desolvation gas (for example nitrogen) flows between the nebulizer 1 and the heater 4 at a flow rate of typically 1200 L/hr.
The high velocity stream of droplets from the nebulizer 1 impact on a 1.6 mm diameter stainless steel, cylindrical rod target 5. Typically, the surface of the rod target 5 is polished and smooth. The illustrated dimensions x1, y1 and y2 are typically 5 mm, 3 mm and 7 mm, respectively. The nebulizer 1 and impactor target 5 are typically held at 0 V and 1 kV, respectively. The mass spectrometer inlet is typically close to ground potential (for example 0-100 V).
A nitrogen curtain gas flow of typically 150 L/hr passes between the cone gas nozzle 6 and the ion inlet cone 10. Ions, charged particles or neutrals that are contained within the gas flow wake 7 from the impactor target 5 can enter the mass spectrometer via the ion inlet orifice 8 which forms a boundary between the first vacuum region 9 of the MS and the atmospheric pressure region of the source enclosure.
When the diameter of the impactor target 5 is significantly greater than the internal diameter of the liquid capillary 2, it is advantageous to direct the spray such that it impacts the target 5 on the upper right hand quadrant, as shown in FIG. 1. Under these conditions, the gas flow wake 7 follows the curvature of the target (Coanda effect) and is swung in the direction of the ion inlet orifice 8 which results in a greater ion signal intensity.
Thus, in an impactor spray source, a nebulizer produces a stream of high velocity liquid droplets in a supersonic gas jet that impinges on a metallic rod target that is held at high voltage and is in close proximity to the nebuliser tip.
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